Malware will often try to block security software from running, and one of the ways it does this is by adding their code signing certificate to Windows' "Untrusted" list.

If you try to run a program with an untrusted certificate, Windows displays a message like "this program has been blocked for your protection", or "an administrator has blocked you from running this program". (These messages can also appear for other reasons.)

AVCertClean is a simple program from AdwCleaner and Malwarebytes developer Jerome Boursier which scans the Untrusted Certificate list, and removes any items which belong to trusted antivirus providers.

The process is almost instant. A text log is created with a report and opened in your default plain text viewer (Notepad, probably). If nothing has happened, you'll see a message telling you no certificates were found in the untrusted list. If the program has removed something, it'll tell you exactly what, and if you close the file and reboot you should be able to install your security software.

Please note: while AVCertClean is easy to use, it's also possible to do the same thing without using any third-party software. Run certmgr.msc, browse to Untrusted Certificates and check any folder under that. Look for certificates which relate to the security software you can't run. If you see anything, deleting it might solve the problem, though keep in mind that deleting the wrong certificate will also make you less secure. Don't do anything unless you're confident it's the right decision.